There are a number of reasons why it is easier to kill cancer cells in culture than in solid tumors. One that has received relatively little attention is the problem of poor microscopic access. For a number of years, we have been exploring ways to integrate macroscopic and microscopic aspects of the pharmacology of biologically interesting ligands, principally to get at the problem of poor access. That work has centered on monoclonal antibodies but with an eye to correlates in the pharmacology of other biological ligands and low molecular weight agents. Theoretical: We first developed a CRAY supercomputer program package (PERC) that solves the differential equations for macroscopic and microscopic pharmacology. PERC has generated a number of unexpected predictions and assisted in the design of experiments. We formulated the "binding site barrier" hypothesis - i.e., that the very fact of successful binding to a target antigen, receptor, or transporter can limit penetration into the substance of a tumor. Calculations suggested that (i) the barrier effect could prevent penetration even 100-200 microns from a blood vessel; (ii) paradoxically, high affinity and high target density could lead to lower concentrations of ligand a few hundred microns from a vessel; (iii) increasing the dose of ligand involves a delicate balance of effects. We have now extended these calculations to two-step therapy, in which a slowly distributing ligand is chased by a fast-distributing ligand-effector chimera. Experimental: We recently obtained direct experimental verification of the binding site barrier hypothesis. With R. Neumann, et al. in the NM, CC, we examined bulk tumor and micrometastases of L10 carcinoma in guinea pigs. A combination of double-label autoradiography and double-chromophore immunohistochemistry detected simultaneously the distribution of antibody, control IgG, antigen, and blood vessels. Results in both s.c. tumors and micrometastases fit the hypothesis. The "binding site barrier" effect is probably a factor in the evolution of autocrine/paracrine molecules. As a corollary, the micropharmacology should be considered when designing molecules for exogenous administration or for secretion by genetically modified cells in viva Current work centers on the access problem in a new hollow fiber model for solid tumors that we are developing in collaboration with M. Hollingshead, J. Mayo, et al. (see Proj. #Z01 CM 07349-01).